If there was ever a poster child for the notion that money can’t buy you happiness, Casey Johnson was it.

Being born into a life of extreme affluence with nearly endless options, Johnson was the classic poor little rich girl — beset with health problems and inner demons that ultimately overwhelmed her.

As the daughter of Woody Johnson, billionaire owner of the New York Jets, and heiress to the vast Johnson & Johnson family fortune, Casey had so much, she didn’t know what to do with it.

“I have so much stuff that, you know, it’s almost embarrassing, it really is,” she told Page Six Magazine in 2008. “I got my first Chanel bag at 12. I got a $17,000 gold Cartier watch when I was 15.”

Still, she hinted that there was a dark side to too much money.

“There’s nothing left to want,” she said.

Those close to her said Johnson was miserable for much of her 30 years — battling a debilitating case of juvenile diabetes she was diagnosed with at 8 and never truly becoming comfortable in her own skin.

“Her problems went way beyond drugs, alcohol and diabetes,” said one friend. “It was also mental and emotional problems. She had this destructive inability to cope. She would say, ‘I have problems in my head.’ ”

A family source largely blamed her diabetes.

“I cannot stress enough this idea between mental health and depression being linked to auto-immune issues,” the source said. “For her, depression was a day in, day out issue of dealing with diabetes.”

Johnson’s troubles came to a head just before New Year’s. She was last heard from, via Twitter, on Dec. 29, and was found dead in her Los Angeles home six days later. The cause of death has yet to be determined.

It was a stunning decline. When Johnson first appeared on the scene a decade ago, she impressed veteran socialite-watchers as what one called a “sweet, gorgeous kid.”

She had graduated from a top Manhattan private school and been accepted to Brown University — but then dropped out in her freshman year to take a job interning with PR queen Lizzy Grubman, Gawker.com reported.

She soon gravitated to the fast-moving set — becoming BFFs with fellow blond heiresses Paris and Nicky Hilton and hitting the nightlife scene with a vengeance. There wasn’t a club banquette she wouldn’t dance on or a paparazzi she wouldn’t pose for.

But while Paris managed to turn her partying into a brand and Nicky chose a quieter life, Johnson appeared to never learn how to switch gears, and began a decade-long run as a gossip-page wild child.

“I [saw] the changes,” said Joan Jedell, publisher of Hampton Sheet magazine. “When I first saw her, she was this sweet, gorgeous kid. But over the years, this beautiful girl had just spiraled downward. It was very sad.”

One of her biggest regrets, Johnson once said, was that she had never agreed to join Paris in her hit reality show, “The Simple Life.” Johnson had reportedly been Hilton’s first choice as a partner but turned it down. Hilton then turned to another pal, Nicole Ritchie, who became a star as a result of her participation.

Making matters worse, Johnson had always kept her family at arm’s length, pals said.

She hadn’t spoken to her father for seven years when she died, one said.

But a family source insisted that simply was “not true,” adding the pair’s relationship was “strained, but not broken.”

Her father was a huge supporter of diabetes research, pouring millions into the cause, largely all in an effort to help his little girl.

Still, Casey couldn’t hide that she was upset by the distance between them.

When asked by Page Six Magazine about her fractured relationship with her father, she started to tear up, saying, “It’s not like I got arrested for prostitution or a DUI . . . I didn’t do anything.”

She also had been fighting with her mother, Sale Johnson, for months over custody of her adopted daughter, Ava.

When talking about raising her daughter, Casey said she planned to do things differently than her parents, especially when it came to money.

“Nobody taught me how to handle my money. I don’t want that for Ava,” she said.

Then there’s the infamous feud with her aunt, Libet, whom she accused of stealing her boyfriend, John Dee, in March 2006.

Libet was 56; Casey, 26.

While Sale Johnson long fought to stop Casey from adopting little Ava from the Central Asian nation of Kazakhstan three years ago, several sources close to the family said the mother had still been in denial as to her daughter’s deteriorating condition.

But the possibility that Casey would be found dead at such a young age was something that had been openly discussed among family members, a source said.

“Numerous times, it had mentioned in that house that Casey wasn’t going to live to 35 anyway because she was really sick,” the source said.

Things only got worse when Johnson moved to Los Angeles and fell in with an even wilder crowd. When she tried to adopt, her mother fought her at every step, knowing it was a bad idea.

Soon, Johnson became a regular feature on gossip pages that detailed her fiery relationship with then-girlfriend Courtenay Semel, who once allegedly beat the heiress to a pulp and set her hair on fire.

Johnson later made headlines again when she was arrested for breaking into a pal’s house, stealing thousands of dollars worth of jewelry and clothes and leaving a vibrator in her bed.

By that point, Johnson’s mother had wrested away custody of Ava, and the family took steps to cut her off financially, hoping it would drive her to seek help.

Instead, Johnson remained holed up in her rented house, without power, water or gas because she couldn’t pay the bills.

A Johnson family source insisted that “stories of Casey’s drug-taking have been really overblown” and that her diabetes was to blame for many of her woes.

But in Johnson’s final months, friends said, she was downing industrial amounts of prescription painkillers like OxyContin, Klonopin and Adderall, as well as other illicit substances. Despite claiming to be in love with her new gal pal, rubber-faced reality star Tila Tequila — to whom she had just gotten “engaged” — Casey was clearly miserable.

“I don’t know if this was an accident and she didn’t mean to die,” said a pal. “She took enough drugs every day to kill her.”